This invention relates to circuit breaker assemblies with a thermal magnetic trip unit and, more particularly, to circuit breakers utilizing an armature for latching a thermal magnetic trip unit.
Residential circuit breakers are described in U.S. Pat. No. 4,513,268 entitled "Automated Q-Line Circuit Breaker". The circuit breaker includes a thermalmagnetic trip unit that interrupts the circuit current upon occasion of so-called "instantaneous", "short time" and "long time" overcurrent conditions. The thermal response of the trip unit is provided by means of an extended bimetal, which is part of the circuit breaker internal current-carrying components, and is separate from the magnet that provides the trip unit magnetic response. The bimetal, in a sense, forms the primary winding of a current transformer with the magnet acting as the transformer core. A separately arranged armature unit responds to the magnetic flux generated by the magnet upon the occurrence of intense overcurrent faults to release the circuit breaker operating mechanism and thereby interrupt the circuit current.
U.S. Pat. No. 4,698,903, entitled "Circuit Breaker High Speed Assembly", describes a drawback to efficient high speed circuit breaker manufacture. Such a drawback is the time-consuming polishing process required on the latching surfaces. The polishing is required to minimize the amount of tripping force that must be applied to overcome the bias of the operating spring and the static friction of the latch surfaces. Although the polishing can be done in a separate pre-assembly process without affecting the actual circuit breaker assembly operation, the trip force required to overcome the mechanism spring bias and the latch surface friction depends to a certain extent upon the polishing operation. The latch surfaces are fabricated from stamped metal parts which exhibit a rough burr on the edge of one surface and a smooth die roll on the edge of the opposite surface. In the prior art, an opening through the stamped part is formed during the stamping operations. Thus, a die break is produced when the slug pushes into the die during the stamping operation and can tear the work material and consequently produce a rough surface. A rough surface is not suitable for latching as it leads to unpredictable performance between the latching surfaces and is a high friction area. Further, high friction does not lend itself to permit smooth and quick disengagement of the latched part from the latch surface.
In an attempt to reduce the primary latch friction, without requiring either polishing or shaving during the circuit breaker assembly operation, a highly polished shim insert was positioned within the armature-latch arrangement in an offline assembly described in the aforementioned U.S. Pat. No. 4,513,268. The insert was in the form of a highly polished stainless steel shim that was welded or brazed within the cradle retaining slot formed in the armature-latch component. It would be economically advantageous to eliminate the on-line shaving process and to eliminate the off-line polished shim insert without affecting the circuit breaker trip response.
Therefore, it is desirable to provide an armature latch that substantially reduces the static friction existing between the latch surfaces without requiring any shaving or shim insertion operations whatsoever.